var canvas = document.getElementById("canvas");
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
// Initialize the GL context
var gl = canvas.getContext('webgl');
if (!gl) {
    console.error("Unable to initialize WebGL.");
}
//Time step
var dt = 0.015;
//Time
var time = 0.0;
//************** Shader sources **************
var vertexSource = `
attribute vec2 position;
void main() {
    gl_Position = vec4(position, 0.0, 1.0);
}
`;

var fragmentSource = `
precision highp float;

uniform float width;
uniform float height;
vec2 resolution = vec2(width, height);

uniform float time;

#define POINT_COUNT 8

vec2 points[POINT_COUNT];
const float speed = -0.5;
const float len = 0.25;
float intensity = 0.9;
float radius = 0.015;

//https://www.shadertoy.com/view/MlKcDD
//Signed distance to a quadratic bezier
float sdBezier(vec2 pos, vec2 A, vec2 B, vec2 C){    
    vec2 a = B - A;
    vec2 b = A - 2.0*B + C;
    vec2 c = a * 2.0;
    vec2 d = A - pos;

    float kk = 1.0 / dot(b,b);
    float kx = kk * dot(a,b);
    float ky = kk * (2.0*dot(a,a)+dot(d,b)) / 3.0;
    float kz = kk * dot(d,a);      

    float res = 0.0;

    float p = ky - kx*kx;
    float p3 = p*p*p;
    float q = kx*(2.0*kx*kx - 3.0*ky) + kz;
    float h = q*q + 4.0*p3;

    if(h >= 0.0){ 
        h = sqrt(h);
        vec2 x = (vec2(h, -h) - q) / 2.0;
        vec2 uv = sign(x)*pow(abs(x), vec2(1.0/3.0));
        float t = uv.x + uv.y - kx;
        t = clamp( t, 0.0, 1.0 );

        // 1 root
        vec2 qos = d + (c + b*t)*t;
        res = length(qos);
    }else{
        float z = sqrt(-p);
        float v = acos( q/(p*z*2.0) ) / 3.0;
        float m = cos(v);
        float n = sin(v)*1.732050808;
        vec3 t = vec3(m + m, -n - m, n - m) * z - kx;
        t = clamp( t, 0.0, 1.0 );

        // 3 roots
        vec2 qos = d + (c + b*t.x)*t.x;
        float dis = dot(qos,qos);

        res = dis;

        qos = d + (c + b*t.y)*t.y;
        dis = dot(qos,qos);
        res = min(res,dis);

        qos = d + (c + b*t.z)*t.z;
        dis = dot(qos,qos);
        res = min(res,dis);

        res = sqrt( res );
    }

    return res;
}


//http://mathworld.wolfram.com/HeartCurve.html
vec2 getHeartPosition(float t){
    return vec2(16.0 * sin(t) * sin(t) * sin(t),
                            -(13.0 * cos(t) - 5.0 * cos(2.0*t)
                            - 2.0 * cos(3.0*t) - cos(4.0*t)));
}

//https://www.shadertoy.com/view/3s3GDn
float getGlow(float dist, float radius, float intensity){
    return pow(radius/dist, intensity);
}

float getSegment(float t, vec2 pos, float offset, float scale){
    for(int i = 0; i < POINT_COUNT; i++){
        points[i] = getHeartPosition(offset + float(i)*len + fract(speed * t) * 6.28);
    }

    vec2 c = (points[0] + points[1]) / 2.0;
    vec2 c_prev;
    float dist = 10000.0;

    for(int i = 0; i < POINT_COUNT-1; i++){
        //https://tinyurl.com/y2htbwkm
        c_prev = c;
        c = (points[i] + points[i+1]) / 2.0;
        dist = min(dist, sdBezier(pos, scale * c_prev, scale * points[i], scale * c));
    }
    return max(0.0, dist);
}

void main(){
    vec2 uv = gl_FragCoord.xy/resolution.xy;
    float widthHeightRatio = resolution.x/resolution.y;
    vec2 centre = vec2(0.5, 0.5);
    vec2 pos = centre - uv;
    pos.y /= widthHeightRatio;
    //Shift upwards to centre heart
    pos.y += 0.02;
    float scale = 0.000015 * height;

    float t = time;

    //Get first segment
    float dist = getSegment(t, pos, 0.0, scale);
    float glow = getGlow(dist, radius, intensity);

    vec3 col = vec3(0.0);

    //White core
    col += 10.0*vec3(smoothstep(0.003, 0.001, dist));
    //Pink glow
    col += glow * vec3(0.94,0.14,0.4);

    //Get second segment
    dist = getSegment(t, pos, 3.4, scale);
    glow = getGlow(dist, radius, intensity);

    //White core
    col += 10.0*vec3(smoothstep(0.003, 0.001, dist));
    //Blue glow
    col += glow * vec3(0.2,0.6,1.0);

    //Tone mapping
    col = 1.0 - exp(-col);

    //Output to screen
    gl_FragColor = vec4(col,1.0);
}
`;

//************** Utility functions **************

window.addEventListener('resize', onWindowResize, false);

function onWindowResize() {
    canvas.width = window.innerWidth;
    canvas.height = window.innerHeight;
    gl.viewport(0, 0, canvas.width, canvas.height);
    gl.uniform1f(widthHandle, window.innerWidth);
    gl.uniform1f(heightHandle, window.innerHeight);
}


//Compile shader and combine with source
function compileShader(shaderSource, shaderType) {
    var shader = gl.createShader(shaderType);
    gl.shaderSource(shader, shaderSource);
    gl.compileShader(shader);
    if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
        throw "Shader compile failed with: " + gl.getShaderInfoLog(shader);
    }
    return shader;
}

//From https://codepen.io/jlfwong/pen/GqmroZ
//Utility to complain loudly if we fail to find the attribute/uniform
function getAttribLocation(program, name) {
    var attributeLocation = gl.getAttribLocation(program, name);
    if (attributeLocation === -1) {
        throw 'Cannot find attribute ' + name + '.';
    }
    return attributeLocation;
}

function getUniformLocation(program, name) {
    var attributeLocation = gl.getUniformLocation(program, name);
    if (attributeLocation === -1) {
        throw 'Cannot find uniform ' + name + '.';
    }
    return attributeLocation;
}

//************** Create shaders **************

//Create vertex and fragment shaders
var vertexShader = compileShader(vertexSource, gl.VERTEX_SHADER);
var fragmentShader = compileShader(fragmentSource, gl.FRAGMENT_SHADER);

//Create shader programs
var program = gl.createProgram();
gl.attachShader(program, vertexShader);
gl.attachShader(program, fragmentShader);
gl.linkProgram(program);

gl.useProgram(program);

//Set up rectangle covering entire canvas 
var vertexData = new Float32Array([-1.0, 1.0, // top left
    -1.0, -1.0, // bottom left
    1.0, 1.0, // top right
    1.0, -1.0, // bottom right
]);

//Create vertex buffer
var vertexDataBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertexDataBuffer);
gl.bufferData(gl.ARRAY_BUFFER, vertexData, gl.STATIC_DRAW);

// Layout of our data in the vertex buffer
var positionHandle = getAttribLocation(program, 'position');

gl.enableVertexAttribArray(positionHandle);
gl.vertexAttribPointer(positionHandle,
    2, // position is a vec2 (2 values per component)
    gl.FLOAT, // each component is a float
    false, // don't normalize values
    2 * 4, // two 4 byte float components per vertex (32 bit float is 4 bytes)
    0 // how many bytes inside the buffer to start from
);

//Set uniform handle
var timeHandle = getUniformLocation(program, 'time');
var widthHandle = getUniformLocation(program, 'width');
var heightHandle = getUniformLocation(program, 'height');

gl.uniform1f(widthHandle, window.innerWidth);
gl.uniform1f(heightHandle, window.innerHeight);

function draw() {
    //Update time
    time += dt;

    //Send uniforms to program
    gl.uniform1f(timeHandle, time);
    //Draw a triangle strip connecting vertices 0-4
    gl.drawArrays(gl.TRIANGLE_STRIP, 0, 4);

    requestAnimationFrame(draw);
}

draw();


